Generally, a switching-mode power converter converts an input voltage to an output voltage by alternatively switching a pair of high-side and low-side elements coupled with the input voltage. In a synchronous power converter, the high-side and low-side elements both are switches such as MOS transistors, and in an asynchronous power converter, the high-side and low-side elements are a switch and a diode, respectively. Additionally, power converters are classified into two types, boost converter and buck converter, depending on the way of producing the output voltage by boosting or bucking the input voltage.
In most portable electronic products, alkaline and lithium batteries are used for power source. However, the battery voltage gradually decays as it is used. To obtain a stable supply voltage, two-stage power converter is generally used to convert the battery voltage to the supply voltage in such a way that boosts the battery voltage to a higher voltage and then bucks it to the desired level.
Conversion efficiency is an important factor to evaluate the performance of a power converter. FIG. 1 shows a block diagram of a conventional two-stage power converter 100, which comprises a boost converter 102 and a buck converter 104 coupled in series. The input voltage Vin of the power converter 100 is provided by one lithium battery or two alkaline batteries, and in the power converter 100, the boost converter 102 steps up the input voltage Vin to produce a higher voltage Vout1, and then the buck converter 104 steps down the voltage Vout1 to produce the output voltage Vout2 at the desired level. However, either one of boost converter and buck converter typically has a conversion efficiency of about 90%, and thus a two-stage power converter will have a lower conversion efficiency not greater than 81%. As a result, the total conversion efficiency is lowed down after the two-stage power conversion.
Therefore, it is desired a high efficiency power converter.